1. Field of the Invention
The invention relates to a production method of a polyimide solution and a fluorinated polyimide solution.
2. Description of the Related Art
Polyimide has been utilized widely as an optical material, a wiring substrate material, a photosensitive material, a liquid crystal material, or the like owing to the properties such as the excellent heat resistance and mechanical strength. Accordingly, it is one of important resin materials.
However, from another point of view, polyimide is difficult to be molded attributed to the excellent properties as a material. That is, since polyimide is un-fusible and insoluble in solvents, it is hard, for example, to dissolve it in a solvent and form it in a film or the like. Therefore, it is common that a polyamide acid, a precursor of polyimide, is synthesized from an acid dianhydride and a diamine compound; using the obtained solution, molding is carried out; and after that the molded polyimide product is obtained by heating or using a chemical method.
However, it is needed to carry out heating at a high temperature, generally as high as 250 to 400° C. for several hours or more, for polyimidation of a polyamide acid. As a result, the resin sometimes becomes more. Such coloration becomes a problem particularly in the case where polyimide is used for an optical material, and the light with near-infrared wavelength necessary for light transmission is decreased during information transmission in some cases. Further, an optical waveguide is composed of a core and a clad respectively made of materials having different light refractive indexes, and if polyimide is used for one of the materials, materials which are inferior in heat resistance cannot be used as the other material since polyimidation requires high temperature treatment.
On the other hand, if the heating temperature is set to be low, not only the time taken for polyimidation becomes long but also there occurs a problem that the strength of polyimide to be obtained is lowered. That is, if the heating temperature of a polyamide acid is gradually increased, the molecular weight of the polyamide acid is decreased at a temperature around 150 to 200° C., and re-bonding of molecules and polyimidation occur at a further higher temperature. Accordingly, if polyimidation is carried out at a relatively low temperature for a long time, only polyimide having a low molecular weight and a low strength can be obtained.
As another method of polyimidation of a polyamide acid solution, there is a method of causing azeotropy of water generated at the time of polyimidation by adding xylene, toluene or the like to a polyamide acid solution. The method makes polyimidation at a relatively low temperature, generally about 80 to 200° C., possible. However, the method is inefficient industrially, because it requires an additional step of adding xylene or the like, and further, it takes a relatively long time to achieve a high imidation ratio.
On the contrary, a chemical polyimidation method is capable of carrying out polyimidation at a relatively low temperature within a short time and thus efficient. However, the polyamide acid solution itself is unstable and there still remains a problem that the molecular weight of a polyamide acid becomes low. Recently, it is proved that such a phenomenon occurs even in the absence of water. Also, since the viscosity of a polyamide acid solution is high, it is difficult to evenly mix a dehydration cyclization agent for polyimidation. Such a problem particularly becomes outstanding in large scale and industrial execution. On the other hand, if the solvent amount is increased and the viscosity is lowered to make even mixing of the reagent easy, the waste solution amount is increased and it cannot be thus applicable for large scale operation.
Accordingly, techniques of directly obtaining a polyimide solution by dissolving polyimide in a solvent but not obtaining polyimide from a polyamide acid solution have been developed. If such a polyimide solution is used, a polyimide product can be produced by simply removing a solvent without requiring treatment at an excess high temperature.
For example, Japanese Unexamined Publication No. 5-17576 discloses soluble aromatic polyimide. According to the examples, it is said that the polyimide can be dissolved in a concentration of 15% in N,N-dimethylformamide or the like.
However, in the examples of this prior art, a polyamide solution is chemically polyimidated to obtain a polyimide powder. Herein, although detailed conditions of the scale of the example are not written, it becomes more difficult to evenly mix a dehydration cyclization reagent in a highly viscous polyamide acid solution as the scale becomes bigger. Therefore, the technique is supposed to be unsuitable for large scale execution. Also, in this technique, once powdered polyimide is dissolved in a solvent to obtain a solution, which is a technique for specified polyimide having high solubility. Generally, as the concentration of polyimide is increased more, the viscosity of the solution is also increased more, and a common stirrer cannot deal with the solution.
Also, Japanese Unexamined Publication No. 3-62868 discloses polyimide varnish for an optical material. The varnish is obtained by dissolving a polyamide acid obtained by reaction of a specified tetracarboxylic acid anhydride and diamine compound, and imidated in a range of imidation ratio from 20 to 98% in a solvent. However, in this technique, the polyamide acid solution is heated at a relatively low temperature from 70 to 250° C. to be imidated, and therefore, it is supposed that the molecular weight is decreased.
Not to change the subject, but Japanese Unexamined Publication No. 7-149896 discloses a method of producing a polyamide acid solution by mixing an oligomer solution of a tetracarboxylic acid and a diamine component solution by a rotation-revolution type mixing method. However, the method aims to produce solely a polyamide acid solution and there is no description of a polyimide solution.
Recently polyimide having fluorine in substituent groups has drawn attention as polyimide having sufficient heat resistance to fabricate an optoelectronic integrated circuit and a low light transmission loss in the near infrared region, particularly the optical communication wavelength region (1.0 to 1.7 μm) and thus suitable for an optical material. For example, Japanese Unexamined Publication No. 5-1148 discloses perfluorinated polyimide in which all C—H bonds are substituted with C—F bonds.
However, in the examples of this prior technique, polyimide is obtained by applying a solution of a polyamide acid as a precursor to an aluminum plate by spin coating, and removing the solvent and firing by heating at 70 to 350° C. for several hours. However, in the case of such a method, the light transmission loss of the polyimide particularly in a wavelength region of 1.0 μm or shorter is often high. In these years, with respect to optical materials, particularly excellent optical properties have been required, and therefore the method for carrying out polyimidation by heating at a high temperature may possible result in an adverse consequence.
On the other hand, as disclosed in Japanese Unexamined Publication No. 5-1148, if polyimide is dissolved in a solvent to produce varnish and the varnish is used for producing an optical waveguide or the like, heating is required simply to remove the solvent and accordingly, it may be possible to produce high quality products. However, in the case of polyimide obtained from a polyamide acid solution by heating at a high temperature, it is supposed that crosslinking reaction among molecules takes place due to high temperature firing, and thus such a polyimide is completely insoluble in a solvent.